Femtosecond laser apparatus and femtosecond laser system including the same

ABSTRACT

There is disclosed a femtosecond laser apparatus including a first laser material comprising Ng, Np and Nm axes spatially perpendicular to each other; a second laser material comprising Np axis, Nm axis and Ng axis; and a first laser diode and second laser diodes, wherein the traveling direction of laser beams generated from the first and second laser materials is substantially parallel to Ng axis of the first laser material and the polarizing direction of laser beams generated from the first and second laser materials is substantially parallel to Np axis of the first laser material, and the traveling direction of laser beams generated from the first and second laser materials is substantially parallel to Np axis of the second material and the polarizing direction of laser beams generated from the first and second laser materials is substantially parallel to Nm axis of the second laser material.

FIELD

The present invention relates to a femtosecond laser apparatus, moreparticularly, to a femtosecond laser apparatus that is able to enhancean output power of laser and a quality of pulse beam, and a femtosecondlaser system including the same.

BACKGROUND

A femtosecond laser light source generates an ultrashort pulse having ahigh peak power and has a high average output of generated pulses.Examples of such a femtosecond laser light source include a femtosecond(fs) laser pulse. The femtosecond laser light sources have been broadlyapplied not only to basic science fields including ultrahigh speedaspectrochemistry, high energy physics, XUV-wave generation and the likebut also to a variety of fields including microprecision laserprocessing, micro-surgery.

Generally, the femtosecond laser pulse has good properties. Examples ofthe good properties include a short pulse time width, a high peakingpower and a broad spectrum bandwidth.

Such the femtosecond laser pulse may be applied to micro or nanoprocessing of electronic components and optical components requiringultraprecision. Examples of such electronic and optical componentsinclude a solar cell, an optical memory, a semiconductor and a flatpanel display. Accordingly, demands on femtosecond pulse laser systemfor industry have been increasing.

To meet such demands on the femtosecond pulse laser system, conditionsfor applying the femtosecond laser pulse to the ultra precision laserprocessing will be described.

First of all, a laser pulse time width is much shorter than anelectronphonon relaxation time of an object not to transmit a thermalenergy generated in processing to a portion near a portion that will beprocessed (non-thermal processing).

That is called as cold ablation.

For example, the electronphonon relaxation time of aluminum is 4.27picosecond (hereinafter, ps) and that of iron is 3.5 ps and that ofcopper is 57.5 ps.

Specifically, when femtosecond laser processing aluminum, laser pulseare applied to aluminum in a pulse time width that is a picoseconds orless for cold ablation.

Accordingly, the femtosecond laser is proper to the ultraprecision laserprocessing of cold ablation.

A femtosecond laser pulse in a femtosecond range can minimize thermaldiffusion in a processing region and not cause the damage generated bylatent heat. Accordingly, the femtosecond laser can process quite a hardmaterial and have a short pulse time width as well as high pulse energy,with a high peak power, only to be advantageous to a nonlinear opticaleffect, namely, multi-photon absorption. Accordingly, the femtosecondlaser can perform various nanometer scaled ultra precision processing ofvarious materials. Examples of such various materials include glass,polymer and even transparent materials.

Second, processing object materials have an ablation threshold valuesthat are approximately several J/cm² or more. Considering the size ofthe laser beam concentrated on a processing area for ablationprocessing, a pulse energy of approximately 10 μJ is required.

In some cases of material processing application can require hundreds ofμJ pulse energy.

An exemplary one of lasers having those good properties may be titaniumsapphire laser (Ti: sapphire laser).

Until now, a commercially useful Ti sapphire laser may provideapproximately several to hundreds of femtosecond pulse time width,several mJ or several J pulse energy.

However, in a conventional femtosecond laser apparatus, a high-pricedhigh output pulse green laser such as Nd:TVO₄ laser has to be used as apumping laser source and it is difficult to gain dozens of kHz or morepulse repetition rate.

Also, the Ti sapphire laser has a large scale system and a high priceand also it is difficult to maintain a pulse power stably and it is noteasy to use the Ti sapphire laser in a production worksite.

Meanwhile, a diode-pumped solid-state (DPSS) laser uses a micro-sizedlight source as a pump beam source. Examples of such a micro-sized lightsource include a laser diode. As a femtosecond laser is configurated byusing a solid laser material, an optical pumping structure can be simpleand the size of a laser head can be small. Also, a laser diode at awavelength commercially used in various fields has a relatively lowprice in comparison to the power thereof and the price of thefemtosecond laser can be reduced, such that an effect of cost reductioncan be gained.

In addition, the solid laser has a short optical pumping distance and itcan perform stable laser operation, such that it can be advantageouslyapplied to a laser for industrial usage.

Recently, with development of semiconductor and electronic technologies,a laser diode array, a laser diode bar and the like have been developedthat are able to perform high power, with small sizes and highefficiency, and development of solid laser systems using diode pumpinghave been growing rapidly.

To realize the femtosecond laser system that is able to optically pump,using a laser diode, it is necessary to select a laser material meetingpredetermined conditions and to design and fabricate an optical pumpingmodule for performing the optical pumping.

Crystals doped with rare-earth ions that can perform diode pumping at808 nm and 980 nm, respectively, may be usually used as the lasermaterial for diode pumping. Examples of such crystals doped withrare-earth ion may include Neodymium (Nd) and Ytterbium (Yb).

In an early stage of a high power laser developing step, a laser crystaldoped with Neodymium is preferred because of a 4-level structure andvarious absorption lines. In recent, a laser crystal doped withYtterbium has been used a lot because of excellent thermal and opticalproperties.

There are additional necessary conditions required to apply thefemtosecond laser light source to industrial settings including theultraprecision laser processing.

For example, if a pulse repetition rate of a laser is low, it takes muchtime to perform laser processing and productivity of the industrialsettings might deteriorate.

However, it is preferred to heighten the pulse repetition rate of thelaser but it is restricted to height the pulse repetition rate.

If the pulse repetition rate is too high to receive the next laser pulsebefore plasma generated by the femtosecond laser pulse disappears, thenext laser pulse can be ill-affected by the plasma remaining near atarget. For example, a beam traveling direction of the next laser pulsemight be changed or the pulse time width thereof might be changed.

That phenomenon is called as ‘plasma shielding’.

To restrain the plasma shielding, the next laser pulse has to be appliedafter a relaxation time of the former plasma.

In other words, a time interval between one laser pulse and the nextlaser pulse has to be longer than the plasma relaxation time. The plasmarelaxation time may be different for each of processing objectmaterials. However, the repetition rate of the plasma relaxation time isapproximately 1 MHz based on the laser pulse repetition rate.

Accordingly, to maintain the high productivity in industrial settings, afemtosecond laser having a pulse repetition rate at hundreds of kHz isrequired.

In addition, to mount a laser light source to a laser processing systemand to operate the laser processing system, the femtosecond laser isrequired to have a compact size and a low price and a high operationstability that makes a laser operational state not changed for asubstantially long time.

When a femtosecond pulse is generated in mode locking in a femtosecondoscillator initially, a pulse energy of the femtosecond pulse is loweredby a nanojoule (nJ) and it is not appropriate to apply such thefemtosecond pulse to a laser processing.

To enhance the femtosecond pulse energy, Chirped Pulse Amplification(CPA) is used.

For example, a pulse stretcher is used in stretching a pulse generatedfrom a femtosecond oscillator longitudinally and timely and the pulse isapplied to an amplifier to amplify the pulse energy.

Hence, the amplified pulse passes a pulse compressor to restitute a timewidth of the pulse to an initial femtosecond range.

At this time, the pulses generated from the oscillator are employed asseeding pulses applied to the amplifier.

Pulses are timely and longitudinally stretched by a difference ofpassages according to wavelengths generated in the pulse stretcher,which can be called as ‘chirping’ and technology for amplifying thepulse energy is called as ‘chirp pulse amplification technology’.

When using such chirp pulse amplification technology, a peaking power ofthe pulse amplified in a resonance cavity of the pulse amplifier keptlow and non-linear distortion generated in spatial or temporaldistribution of the laser pulses can be retrained. Moreover, a physicaldamage applied to optical components composing the system can beprevented.

Specifically, the pulse amplifier can be operated to prevent the damageto the system generated by the high energy laser pulse and to enhancethe pulse energy.

Recently, the high pulse energy has been gained from MOPA systemcombined with Maser Oscillator (MO) and Power Amplifier (PA) and a hugestep has made in development of femtosecond laser systems having a highpeaking power and a high average power accordingly. The masteroscillator directly pumps a diode light source based on the chirp pulseamplification technology and the power amplifier directly pumps thediode light source.

However, the laser material doped with Ytterbium has a 2-level energystructure or a 3-level energy structure and it has a disadvantage thatlights emitted at an optical pumping wavelength of 981 nm is absorbed bythe laser material again.

To solve the disadvantage, the light generated from a high brightnesslaser diode having a high power is focused on the laser crystal with amicro-spot size.

In this process, pump beams failed to be converted into laser beams aretransmitted adjacent to the spot of the laser crystal in a thermalenergy type and to a mount where the laser crystal is mounted.

When the thermal energy is collected largely, amplified laser beams arecrushed and the quality of beams might deteriorate. Also, an averagepower of lasers and pulse energy might be restricted.

After that, the thermal energy focused on the laser crystal is higherthan a damage threshold, the laser crystal might have physical damage,for example, crack or break only to stop laser oscillationproblematically.

Meanwhile, conventional studies on generation or amplification offemtosecond pulses by using a plurality of Yb:KYW or Yb:KGW lasercrystals will be described as follows.

For example, U.S. Pat. No. 7,508,847 B2 discloses a new concept ofincreasing the frequency of passages of laser beams via gain material ina resonance cavity by pumping two anisotropic materials includingYb:KGW.

However, in the application, the length of the resonance cavity isincreased to reduce an instable phenomenon of a pulse power called as“Triggered mode”. It is proposed that a pumping structure of two gainmaterials using one of various types of long resonance cavities.However, that application fails to experimental embodiments and results.

In addition, U.S. Pat. No. 6,760,356 B2 discloses a method of amplifyinga femtosecond pulse by using two anisotropic laser crystals of Yb:YAG.

However, those two prior patent applications disclose only the poweramplification by using only two laser materials and fail to disclosethermal, optical and other various characteristics that have to be putinto consideration of an optical axis of a laser material in the otherfemtosecond laser systems. In other words, the two patent applicationsdisclose only an effect of the output power enhanced by increasing thenumbers of the laser materials, with no comments on the axis of thelaser material.

Meanwhile, U.S. Pat. No. 6,891,876 B2 discloses a study on absorbingdepolarized pump beams by selecting an axis of an Yb:KYW laser materialand controlling a polarization rate of lights pumping a laser material.

The application focuses on light pumping of an anisotropic material.Even in case of using a first depolarized pump beam, an axis of ananisotropic gain material and a wavelength of a pumping laser areselected to optically pump the laser material efficiently.

The application discloses that even in case the wavelength of a secondpump beam is instable, an axis of the laser material is selected tooptically pump the laser material efficiently.

The proposed method uses an absorption spectrum that is differentiatedaccording to an axis direction of the laser material and properlydetermines a direction of the laser material based on the differentabsorption spectrums to properly mix two axes of one laser material.When the intensity of the pump beam is controlled according to thepolarization direction, similar absorption cross-sections are shown inbroad wavelength region.

However, in this instance, there is an advantage of providing a lightpumping unit less sensitive to instability of polarization or wavelengthof the pump beam. However, to have similar absorption cross-sections inbroad wavelength regions, an absorption cross-section of the lasermaterial has to perform light pumping in a wavelength region having asmall absorption cross-section of the laser material.

Moreover, the output has to be dispersed to two polarized lights fromone laser light source. Because of that, pumping efficiency coulddeteriorate badly and the pump beams absorbed after failed to beconverted into laser wavelengths are focused on the laser material bythe terminal energy, such that the quality of the laser beam maydeteriorate and the power of the laser beam may be restricteddisadvantageously.

The Yb:KYW or Yb:KGW laser crystal has a good heat conduction qualityand it has an advantage of generating a high average power femtosecondlaser. However, the conductivity of the anisotropy laser material suchas Yb:KYW or Yb:KGW is differentiated according to the axis direction.If the average power of the laser is high, astigmatism of a thermal lensmight be generated by a thermal effect and the shape of the laser beamis distorted to deteriorate the quality of the laser beam.

In addition, while generating or amplifying the femtosecond pulse byusing the plurality of the laser crystals, the pump beam generated fromthe laser diode is incident on and absorbed to one of the laser crystalsand the other pump beams not absorbed to the laser crystal areirradiated to optical components or optical mounts. Accordingly, thealignment of the pump beams and laser beams might deteriorate.

DETAILED DESCRIPTION OF THE INVENTION Technical Problem

To solve those disadvantages, an object of the invention is to provide afemtosecond laser apparatus that can enhance a quality and a powerintensity of beams, without beam distortion by using a plurality oflaser materials and making laser beams generated from the lasermaterials be substantially parallel to a specific axis of the lasermaterials, and a femtosecond laser system including the same.

Another object of the present invention is to provide a femtosecondlaser apparatus that is able to restrain gain narrowing generated whileamplifying a pulse spectrum-shaped in an extra-cavity as a seeding pulseso as to shorten the time width of a pulse emitted from a laser systemincluding the same, and a femtosecond laser system including the same.

A further object of the present invention is to provide a femtosecondlaser apparatus that is able to broaden a bandwidth of a gain spectrumby making a polarizing direction of laser beams generated from lasermaterials be substantially parallel to a specific axis of each lasermaterial, and a femtosecond laser system including the same.

A still further object of the present invention is to provide afemtosecond laser apparatus that is able to broaden spectrums of apulses by overlapping gain spectrums of laser materials having differentwavelengths corresponding to the maximum value of an emission crosssection, and a femtosecond laser system including the same.

A still further object of the present invention is to provide afemtosecond laser apparatus that is able to prevent an alignmentcharacteristic of pump beams and laser beams generated in a process ofgenerating or amplifying femtosecond pulses from a plurality of lasermaterials when generating femtosecond pulses by using a plurality oflaser materials.

Technical Solution

To achieve these objects and other advantages and in accordance with thepurpose of the embodiments, as embodied and broadly described herein, afemtosecond laser apparatus includes a first laser material comprisingNg, Np and Nm axes spatially perpendicular to each other; a second lasermaterial comprising Np axis substantially parallel to Ng axis of thefirst laser, Nm axis substantially parallel to Np axis of the firstlaser material and Ng axis substantially parallel to Nm axis of thefirst laser material; and a first laser diode and second laser diodesarranged to irradiate pump beams to the first laser material and thesecond laser material, respectively, wherein the first laser material isarranged to make a traveling direction of laser beams generated from thefirst and second laser materials be substantially parallel to Ng axis ofthe first laser material and to make a polarizing direction of laserbeams generated from the first and second laser materials besubstantially parallel to Np axis of the first laser material, and thesecond laser material is arranged to make a traveling direction of laserbeams generated from the first and second laser materials besubstantially parallel to Np axis of the second material and to make apolarizing direction of laser beams generated from the first and secondlaser materials be substantially parallel to Nm axis of the second lasermaterial.

In another aspect of the present invention, a femtosecond laserapparatus includes a first laser material comprising Ng, Np and Nm axesspatially perpendicular to each other; a second laser materialcomprising Np axis substantially parallel to Ng axis of the first laser,Nm axis substantially parallel to Np axis of the first laser materialand Ng axis substantially parallel to Nm axis of the first lasermaterial; and a first laser diode and second laser diodes arranged toirradiate pump beams to the first laser material and the second lasermaterial, respectively, wherein the first laser material is arranged tomake a traveling direction of laser beams generated from the first andsecond laser materials be substantially parallel to Ng axis of the firstlaser material and to make a polarizing direction of laser beamsgenerated from the first and second laser materials be substantiallyparallel to Nm axis of the first laser material, and the second lasermaterial is arranged to make a traveling direction of laser beamsgenerated from the first and second laser materials be substantiallyparallel to Np axis of the second material and to make a polarizingdirection of laser beams generated from the first and second lasermaterials be substantially parallel to Nm axis of the second lasermaterial.

In a further aspect of the present invention, a femtosecond laserapparatus includes a first laser material and a second laser materialcomprising Ng, Np and Nm axes, respectively, that are spatiallyperpendicular to each other, the first laser material and the secondlaser material facing each other; a first laser diode and a second laserdiode arranged to make pump beams incident on the first laser materialand the second laser material, respectively; and an optical componentarranged between the first laser material and the second material tomake a polarizing direction of laser beams generated from the first andsecond laser materials be substantially parallel to Np axis of the firstlaser material and to Nm axis of the second laser material, wherein thefirst laser material is arranged to make a traveling direction of laserbeams generated from the first and second laser materials besubstantially parallel to Ng axis of the first laser material, and thesecond laser material is arranged to make a traveling direction of laserbeams generated from the first and second laser materials besubstantially parallel to Np axis of the second laser material.

In a still further aspect of the present invention, a femtosecond laserapparatus includes a first laser material and a second laser materialcomprising Ng, Np and Nm axes, respectively, that are spatiallyperpendicular to each other, the first laser material and the secondlaser material facing each other; a first laser diode and a second laserdiode arranged to make pump beams incident on the first laser materialand the second laser material, respectively; and an optical componentarranged between the first laser material and the second material tomake a polarizing direction of laser beams generated from the first andsecond laser materials be substantially parallel to Nm axis of the firstlaser material and to Nm axis of the second laser material, wherein thefirst laser material is arranged to make a traveling direction of laserbeams generated from the first and second laser materials besubstantially parallel to Ng axis of the first laser material, and thesecond laser material is arranged to make a traveling direction of laserbeams generated from the first and second laser materials besubstantially parallel to Np axis of the second laser material.

In a still further aspect of the present invention, a femtosecond laserapparatus includes a first laser material and a second laser materialfacing each other; a first laser diode and a second laser diode arrangedto make pump beams incident on the first laser material and the secondlaser material, respectively; and a beam dumper arranged between thefirst laser material and the second laser material, the beam dumpercomprising a hole to pass laser beams generated from the first andsecond laser materials there through and a beam absorbing part formedadjacent to the hole to cut off or absorb the pump beams failed to beabsorbed to the first and second laser materials.

Advantageous Effects

The embodiments have following advantageous effects. According to thefemtosecond laser apparatus according to one embodiment of the presentinvention, the quality as well as the power intensity of the laser beamsgenerated from the laser materials can be enhanced.

Furthermore, a pulse spectrum can be shaped as desired and the timewidth of the pulses may be reduced accordingly.

Still further, the gain spectrums of the laser materials havingdifferent wavelengths, respectively, corresponding to the maximum valueof the emission cross section may be overcalled with each other.Accordingly, the spectrum width may be converted.

Still further, the alignment characteristic of pump beams and laserbeams can be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a femtosecond laser systemaccording to one embodiment of the present invention;

FIG. 2 is a graph to describe a spectrum characteristic as a gain of aamplified and output pulse is narrower according to one embodiment ofthe present invention;

FIG. 3 is a graph to describe spectrum change of an output pulseaccording to spectrum shaping by using a spectral shaper;

FIG. 4 is a diagram illustrating a spectrum characteristic of a emissioncross section possessed by a laser crystal, when a polarizing directionof a laser beam generated from a laser crystal (Yb:KYW) is parallel toNm-axis or Np-axis of the laser crystal (Yb:KYW);

FIG. 5 is a diagram illustrating a spectrum characteristic shown bycombining of a spectrum having a emission cross-section according to oneembodiment of the present invention;

FIG. 6 is a schematic diagram of a laser beam generating apparatusaccording to one embodiment of the present invention;

FIG. 7 is a schematic diagram of a laser beam generating apparatusaccording to another embodiment of the present invention;

FIG. 8 is an optical conceptual diagram of a femtosecond laser apparatusto describe an arrangement structure of a laser material provided in alaser beam generating apparatus of a maser oscillator or an amplifieraccording to one embodiment of the present invention;

FIGS. 9A-9C are diagrams illustrating laser beam characteristicsexperimentally shown according to the arrangement structure of the lasermaterial;

FIG. 10 is an optical conceptual diagram of a femtosecond laserapparatus to describe an arrangement structure of a laser material toenhance an average power of a laser beam according to another embodimentof the present invention;

FIG. 11 is an optical conceptual diagram of another embodiment of thefemtosecond laser apparatus to describe an arrangement structure of alaser material provided in a laser beam generating apparatus of a maseroscillator or an amplifier according to one embodiment of the presentinvention;

FIG. 12 is an optical conceptual diagram of the embodiment of thefemtosecond laser apparatus to describe an arrangement structure of alaser material provided in a laser beam generating apparatus of a maseroscillator or an amplifier according to another embodiment of thepresent invention;

FIG. 13 is a diagram illustrating a laser beam generating apparatusprovided in a master oscillator or amplifier according to one embodimentof the present invention;

FIGS. 14A and 14B are diagrams illustrating beam dumpers according toone embodiment of the present invention;

FIG. 15 is a graph illustrating slope efficiency of a continuous waveaccording to the intensity of a pump beam applied to a laser materialaccording to one embodiment of the present invention;

FIG. 16 is an optical conceptual diagram illustrating a spectrum shapingmachine according to one embodiment of the present invention;

FIG. 17 is a graph illustrating a spectrum before and afterspectrum-shaping a seeding pulse according to one embodiment of thepresent invention;

FIG. 18 is a graph illustrating a spectrum of a pulse amplified and notspectrum-shaping and a pulse amplified and spectrum-shaped according toone embodiment of the present invention;

FIG. 19 is a graph illustrating a pulse time width of a pulse amplifiedand not spectrum-shaping and a pulse amplified and spectrum-shapedaccording to one embodiment of the present invention; and

FIG. 20 is a graph illustrating change in pulse energy according to apulse repetition rate.

DETAILED DESCRIPTION

Reference may now be made in detail to specific embodiments, examples ofwhich may be illustrated in the accompanying drawings. Whereverpossible, same reference numbers may be used throughout the drawings torefer to the same or like parts.

FIG. 1 is a schematic diagram illustrating a femtosecond laser systemaccording to one embodiment of the present invention.

Referring to FIG. 1, a femtosecond laser system 100 may include a masteroscillator 110, a faraday isolator 120, a pulse stretcher/compressor130, a spectral shaper 140, a thin film polariscope (TFP), a faradayrotator 150, an amplifier 170 and a pulse picker 160.

The master oscillator 110 has a laser apparatus configured to generate alaser beam, similar to the amplifier 170, although not shown in thedrawings. The master oscillator 110 may generate a femtosecond pulse ata femtosecond range.

The mater oscillator 110 may use an optical fiber laser or a solid laserto generate the laser beam. Various laser materials used in the solidlaser may be selected and used according to thermal, optical andmechanical characteristics.

For example, the laser material may be a non-crystal material or crystalmaterial. In case of using the crystal material as the laser material,at least one of an isotropic crystal and anisotropic crystal may beselected. Such anisotropic crystal may include a uniaxial crystal andbiaxial crystal.

The non-crystal material may be Yb:Glass and the isotropic crystal ofthe crystal material may be at least one of Yb:TAG, Yb:ScO, Yb:LuO,Yb:LuScO and Yb:CaF. The uniaxial crystal may be at least one ofYb:CALGO, Yb:YVO4, Yb:NGW, Yb:NYW, Yb:LuVO, Yb:LSB, Yb:S-FAP andYb:C-FAP. The biaxial crystal may be at least one of Yb:KYW, Yb:KGW,Yb:KLuW, Yb:YCOB and Yb:YAP.

The pulse stretcher/compressor 130 includes a stretcher 131 and acompressor 132 that are provided therein. The pulse stretcher/compressor130 stretches a femtosecond pulse width generated by the masteroscillator 110 or compresses a pulse amplified by the amplifier into apulse in the femtosecond range again.

The stretcher 131 and the compressor 132 can stretch or compress thepulse by using an auxiliary spectral device. Examples of such a spectraldevice may include a grating.

In addition, the stretcher 131 and the compressor 132 may be integrallyformed with each other to stretch and compress the pulse by sharing onespectral device. Accordingly, the size of the femtosecond laser system100 may be compact and the production cost of the femtosecond lasersystem 100 can be reduced.

The stretcher 131 may temporally stretch the width of the femtosecondpulse generated by the master oscillator 110 and it may prevent physicaldamage to optical components including the laser material in that couldbe generated in a process of the amplifier 170 amplifying thefemtosecond pulse.

For example, the stretcher 131 stretches a pulse of 100 femtosecond (fs)generated by the master oscillator 110 into a pulse of dozens ofpicosecond pulses (ps).

The compressor 132 compresses the pulse amplified by the amplifier 170into a pulse in the femtosecond range and transmits the compressed pulseoutside.

The faraday isolator 120 is arranged between the mater oscillator 110and the pulse stretcher/compressor 130 and it prevents the high energypulse generated by the amplifier 170 from being incident on the masteroscillator 110.

The spectral shaper 140 converts a spectrum of the pulse stretched bythe pulse stretcher 131 as desired. In other words, the spectral shaper140 shapes a spectrum of seeding pulses input to the amplifier 170 andcompensates the spectrum bandwidth narrowed in the amplification processof the amplifier. In this instance, the seeding pulses means the pulsesapplied to the amplifier 170 for pulse amplifying.

Such the spectral shaper 140 may be provided in the master oscillator110 or the amplifier 170 or omitted in the femtosecond laser system 100,in case the mater oscillator 110 or the amplifier 170 converts thespectrum of the pulses as desired.

The spectrum shaping of the pulses will be described later.

The spectrum-shaped pulses are applied to the amplifier 170 afterpassing a full-reflection mirror (FM), the thin film polariscope (TFP)and the faraday rotator 150). At this time, the full reflection mirror(FM) is configured to change a passage of the beam and it can beprovided or omitted according to the size and design conditions of thefemtosecond laser system. In case the full reflection mirror (FM) isused like the embodiment of the present invention, the full reflectionmirror (FM) can change the passage of the beam in a restricted space andthe femtosecond laser system can be compact accordingly.

The amplifier 170 may include a laser beam generating device using anoptical fiber laser or a solid laser, like the mater oscillator 110, andit may amplify the energy of the input seeding pulses by using the laserbeam generating device.

A laser material used in the amplifier 170 may be the same lasermaterial used in the mater oscillator 110 or different from that.

In other words, the laser material used in both of the mater oscillator110 and the amplifier 170 may be various combinations of the crystalmaterial and the non-crystal material. When both of the laser materialsused in the mater oscillator 110 and the amplifier 170 are crystalmaterial, various combinations of the isotropic crystal and theanisotropic crystal may be used.

A specific laser material that can be used in the amplifier 170 issubstantially identical to the laser material that can be used in themater oscillator 110 and detailed description thereof will be omitted.

The faraday rotator 150 is configured to rotate a polarizing directionof the laser pulse to change a passage of the traveling laser pulsesamplified by the amplifier 170 toward the pulse picker 160 via the thinfilm polariscope (TFP).

The pulse picker 160 includes an electro-optic switch. While switchingthe electro-optic switch on and off, the pulse picker 160 distinguishesneeded pulses and from needless pulses to pass them there throughselectively. After that, the selected laser pulses compressed intofemtosecond laser pulses in the femtosecond range by the compressor 132and the femtosecond laser pulses may be emitted outside the femtosecondlaser system.

The femtosecond laser system 100 according to the present inventionamplifies and heightens the energy of the laser pulses in several nJrange generated in the oscillator 110 to make the laser pulses easilyprocessed in the femtosecond laser processing.

Meanwhile, the laser material has a gain profile with a basicallylimited width. As an amplifying width is differentiated according to thewavelength of the pulses, the spectrum bandwidth of the amplified pulsescould be narrowed and this is called as gain narrowing. Accordingly,there might be a problem of a broad time width of the pulses.

FIG. 2 is a graph to describe a spectrum characteristic as a gain of anamplified and output pulse is narrower according to one embodiment ofthe present invention.

Referring to FIG. 2, an input pulse having a spectrum shown in (a) isapplied to a laser material having a gain profile with a limited width.After that, in a process of amplifying the input pulse, amplification iscontinuous in a central wavelength (λc) of the input pulse and a gain islowered in an edge wavelength out of the center and the amplificationrate is smaller in the edge wavelength than in the central wavelength.

In other words, when the input pulse reciprocates in a resonance cavityof the amplifier, the frequency of the input pulse passing the lasergain material is increased and a difference between amplification ratesaccumulates. Accordingly, the intensity of the input pulse is relativelylower in an edge wavelength than in a central wavelength.

As shown in the graph of FIG. 2, the width of the output pulse spectrumshown in (c) is getting narrower than the width of the input pulsespectrum.

When the spectrum bandwidth of the laser pulse is narrower, the timewidth of the laser pulse is wider. Accordingly, the heat is diffused ina mutual action between the laser beam output from the femtosecond lasersystem and a processing product and the processing result of theprocessed products can be deteriorated substantially.

Therefore, according to the embodiment of the present invention, beforethe seeding pulse is applied to the amplifier 170, the spectral shaper140 is provided to shape the pulse to have a desired spectrum, to reducethe time width of the laser pulse. Or, an optical axis of the lasermaterials is changed in the mater oscillator 110 or the amplifier 170 toprevent the spectrum bandwidth from getting narrowed.

In addition, methods for widening the spectrum bandwidth of the pulsemay include a method for widening the spectrum bandwidth of the pulsemay include spatial dispersive amplification, spectrum converting afterinserting an optical device in an resonance cavity, non-linear pulsecompressing.

FIG. 3 is a graph to describe spectrum change of an output pulseaccording to spectrum shaping by using a spectral shaper.

Referring to FIG. 3, spectrum shaping (SS) is performed to lower theintensity of an input pulse in a central wavelength shown in (c) and toheighten the intensity in a M-shape in a wavelength next to the centralwavelength, before applying an input pulse having a spectrum shown in(a) to a laser material having a gain profile with a limited width shownin (b).

When such the pulse having the shaped spectrum is applied to the lasermaterial having the gain profile shown in (b), the gain is low in thecentral wavelength and high in the wavelength near the centralwavelength.

When the laser pulse reciprocates in a resonance cavity of the amplifier170, the frequency of passages through the laser gain material increasesand the differences of amplification rates accumulates only to realizethe output pulse having the output pulse spectrum shown in (d).

The output pulse spectrum (d) is normalized and it is shown that thebandwidth (a) of the output spectrum is widened in comparison to thebandwidth (a) of the input pulse spectrum.

Meanwhile, to prevent the spectrum bandwidth from getting narrow, aplurality of laser materials may be provided in the laser generatingdevice provided in the mater oscillator 110 or the amplifier 170, aswell as the spectrum shaping of the pulse enabled by the spectralshaper. Unique gain spectrums possessed by the plurality of the lasermaterials, respectively, may be spectrally combined.

In other words, a wavelength corresponding to the maximum value of anemission cross-section makes different gain spectrums of the lasermaterials overlapped with each other, such that the spectrum bandwidthmay be widened.

At this time, the laser materials may be one of non-crystal materialsand crystal materials, non-crystal materials and non-crystal materials,crystal materials and crystal materials. The number of the lasermaterials is not limited and the crystal materials may include both ofisotropic crystals and anisotropic materials. To widen the spectrumbandwidth of the pulse, the laser generating device may use the sametypes of laser materials as shown in FIGS. 4 and 5. Usingcharacteristics of the spectrum of emission cross-section beingdifferentiated according to an optical axis, the spectrums of the lasermaterials according to the optical axis are overlapped with each otherand the spectrum bandwidth may be widened.

For example, in case the laser materials are anisotropic crystalsincluding Yb:KYW or Yb:KGW, spectrums of emission cross sections aredifferentiated according to which axial direction a polarizing directionof laser beams generated from the anisotropic crystals is parallel to.

Experimentally, when a polarizing direction of laser beams in awavelength range of 1015˜1050 nm is substantially parallel to Nm axis ofthe laser crystal with respect to optical axes including Nm, Np and Ngof the Yb:KYW laser crystals, the emission cross section is the maximum.The emission cross section is larger when the polarizing direction issubstantially parallel to Np-axis and approximately 10 times smallerwhen the polarizing direction is substantially parallel to Ng-axis, thanwhen it is substantially parallel to Nm-axis.

Such experiments use crystallographic axes including a-axis, b-axis andc-axis in widening the spectrum bandwidth of the pulse, not the opticalaxes.

FIG. 4 is a diagram illustrating a spectrum characteristic of a emissioncross section possessed by a laser crystal, when a polarizing directionof a laser beam generated from a laser crystal (Yb:KYW) is parallel toNm-axis or Np-axis of the laser crystal (Yb:KYW). FIG. 5 is a diagramillustrating a spectrum characteristic shown by combining of a spectrumhaving an emission cross-section according to one embodiment of thepresent invention.

Referring to FIG. 4, when the polarizing direction of the laser beamsgenerated from the laser crystals (Yb:KYW) is substantially parallel toNm-axis of the laser crystal (Yb:KYW), the emission cross section hasthe maximum value near a wavelength of 1025 nm and spectrum distributionof (a) is shown. When the polarizing direction of the laser beamsgenerated from the laser crystals (Yb:KYW) is substantially parallel toNp-axis of the laser crystal (Yb:KYW), the emission cross section hasthe maximum value near a wavelength of 1040 nm and spectrum distributionof (b) is shown.

Referring to FIG. 5, (a) shows that the bandwidth of the spectrum isincreased by combining the spectrums of the emission cross sectionsshown in (a) and (b) of FIG. 4 with each other at 1:1 ratio. (b) showsthat the spectrums of the emission cross sections shown in (a) and (b)of FIG. 4 are combined with each other at 1:3 ratio to increase thespectrum bandwidth of the emission cross section farther.

FIG. shows one of examples for increasing the spectrum bandwidth of theemission cross section. Alternatively, spectral characteristics ofdifferent emission cross sections are combined diversely so as toincrease the spectrum bandwidth. At this time, the spectrum combiningmay uses an optical axis having a large value of emission cross sectionto enhance the power of the laser beams generated from the lasercrystals.

Meanwhile, the emission cross sections and absorption cross sections ofYb:KYW or Yb:KGW laser crystals are different according to a specificone of axial directions. Polarizing directions of pump beams may becombined variously to enhance the light pumping efficiency.

Preferably, an absorption cross section of Ng-axis direction is 10 timessmaller than an absorption cross section of Nm-axis direction and theabsorption direction of Nm-axis direction is approximately 5 timeslarger than that of NP-axis direction, such that the polarizingdirection of the pump beam may be substantially parallel to Nm-axis ofthe laser crystal.

FIG. 6 is a schematic diagram of a laser beam generating deviceaccording to one embodiment of the present invention.

As shown in the drawing, a laser beam generating apparatus 200 may beprovided in the mater oscillator (not shown). The laser beam generatingapparatus 200 may include a laser material (C1 and C2), a dichromaticmirror (DM), a focusing lens (FL), a collimating lens (CL), a first waveplate 24, a laser diode 21, an optical fiber 22 and a beam dumper 30.

Laser diodes 21 are arranged on both sides of the laser material (C1 andC2) and each of the laser diodes 21 is optically connected with theoptical fiber 22. Such the laser diode 21 is a light source configuredto generate a pump beam and it applies the pump beam to the lasermaterial (C1 and C2) via the optical fiber 22.

The type of the laser material (C1 and C2) and the arrangement structureof the laser material (C1 and C2) may be combined variously, consideringspectrum properties, thermal properties, efficiency and power of pulses.

The first wave plate 24 is a half-wave plate (λ/2) and it is arrangednext to the optical fiber 22 along a passage of the pump beam. The firstwave plate 24 may adjust a polarizing direction of the light generatedin the laser diode 21.

The collimating lens (CL) and the focusing lens (FL) are sequentiallyarranged next to the first wave plate 41 along the passage of the pumpbeams and it connect the pump beam polarized in the first wave plate 24to the laser material (C1 and C2).

The dichromatic mirror (DM) reflects the laser beam generated from thelaser material (C1 and C2) and transmits the pump beam generated fromthe laser diode (C1 and C2) there through. For that, the dichromaticmirror (DM) is arranged in front and behind the laser materials (C1 andC2).

Meanwhile, although not shown in the drawing, the first wave plate 24,the collimating lens (CL), the focusing lens (FL), the dichromaticmirror (DM), the laser materials (C1 and C2) may be integrallyfabricated as a light pumping module via mechanical component connectionto allow the pump beam generated from the laser diode to maintain analignment characteristic of the beams incident on the laser materialseven if the temperature or humidity changes, for example.

The beam dumper 30 may be arranged between the laser materials (C1 andC2) to prevent deterioration of the alignment of the pump beams and beamlasers generated in the process of amplifying laser pulses. The beamdumper will be described in detail later.

Moreover, optical components for adjusting the beam passage, forexample, a concave mirror, a convex mirror, a full reflection mirror andthe like are further provided in the laser generating apparatus, toadjust the passage of the laser beam.

FIG. 7 is a schematic diagram of a laser beam generating apparatusaccording to another embodiment of the present invention.

As shown in the drawing, a laser beam generating apparatus 300 may beprovided in an amplifier (not shown). The laser beam generatingapparatus 300 includes a laser material (C1 and C2), a dichromaticmirror (DM), a focusing lens (FL), a collimating lens (CL), a first waveplate 24, a second wave plate 25, a thin film polariscope (TFP), aconcave mirror (CM1 and CM2), a full reflection mirror (FM), a laserdiode 21, an optical fiber 22, a pockels cell 23 and a beam dumper 30.

The laser beam generating apparatus has a structure almost similar tothe structure of the laser beam generating apparatus shown in FIG. 6.Accordingly, detailed description of similar optical components will beomitted.

The concave mirror (CM1 and CM2) and the full reflection mirror (FM) areconfigured to convert a passage of laser beams. The number and positionsof the concave mirror (CM1 and CM2) and the full reflection mirror (FM)may be differentiated according to the scale of the laser beamgenerating apparatus or the traveling distance of the beam passage. Inthe embodiment of the present invention, only the concave mirror and thefull reflection mirror are described as beam passage converting means. Aconvex mirror, a plane mirror and other beam passage converting meansmay be used.

The pockels cell 23, the second wave plate 25 and the thin filmpolariscope (TFP) are arranged in a light passage direction and they areemployed as switches to emit the laser beams generated from the lasermaterial outside. here, the second wave plate 25 is a λ/4-wave plate.For example, several kV voltages are applied to the pockels cell toconvert a polarizing direction of the laser beam amplified via theoptical components.

FIG. 8 is an optical conceptual diagram of a femtosecond laser apparatusto describe an arrangement structure of a laser material provided in alaser beam generating apparatus of a maser oscillator or an amplifieraccording to one embodiment of the present invention.

As shown in the drawing, first and second laser materials (C1 and C2)have crystal faces based on optical axes, respectively, and each of thecrystal faces is formed perpendicular to the optical axis. In otherwords, the first and second laser materials (C1 and C2) have the crystalfaces having Ng, Np and Nm axes spatially perpendicular to each other,respectively. The first laser material (C1) and the second lasermaterial (C2) are aligned differently.

Moreover, the second laser material (C2) may be provided in the laserbeam generating apparatus, with Np-axis substantially parallel toNg-axis of the first laser material (C1), Nm-axis substantially parallelto Np-axis of the first laser material (C1) and Ng-axis substantiallyparallel to Nm-axis of the first laser material (C1).

Although not shown in the drawing, a first and second laser diodes arearranged adjacent to the first laser material (C1) and the second lasermaterial (C2), respectively, to allow the pump beams incident on thefirst and second laser materials (C1 and C2).

The first laser material (C1) and the second laser material (C2) arearranged to allow a traveling direction of the laser beams generatedfrom the first laser material (C1) and the second laser material (C2)substantially parallel to Ng-axis of the first laser material andsubstantially parallel to Np-axis of the second laser material (C2).

As mentioned above, the first laser material (C1) and the secondmaterial (C2) makes the polarizing direction (namely, Elaser) of thelaser beams generated from the first laser material (C1) and the secondmaterial (C2) be substantially parallel to Np-axis of the first lasermaterial (C1) and Nm-axis of the second material (C2), while it isresonating in the laser beam generating apparatus. Accordingly, thelaser beams are traveling in different axial directions of the lasermaterials having different thermal characteristics, such that thequality of the beams can be enhanced, without beam distortion generatedby thermal effects.

In addition, to enhance the efficiency, the first laser material (C1)and the second laser material (C2) may be arranged to make a polarizingdirection (Epump) of first pump beams generated from the first laserdiode be substantially parallel to Nm-axis of the first laser material(C1) and a polarizing direction (Epump) of second pump beams generatedfrom the second laser diode be substantially parallel to Nm-axis of thesecond laser material (C2).

In other words, the polarizing direction of the laser beams generatedfrom the first laser material (C1) and the second laser material (C2)may be substantially parallel to one Nm-axis of the first and secondlaser materials (C1 and C2), considering an emission cross section, andsubstantially parallel o the other Np-axis. Accordingly, the spectrumbandwidth may be broadened and the beam distortion generated by thethermal effect may be prevented. Also, the polarizing direction (Epump)of the pump beams generated from the first and second diodes may besubstantially parallel to Nm-axes of the first and second lasermaterials (C1 and C2), with the maximum absorption cross section, onlyto enhance the efficiency.

According to one embodiment of the present invention, two lasermaterials are provided to firstly complement the thermal characteristicsof the laser materials to enhance the beam quality. Different gainspectrum distributions are combined to broaden the bandwidth. However,three or more laser materials may be provided to broad the bandwidthfarther to reduce the time width of the pulses. The arrangement of thelaser materials and the laser system including the laser materials maybe varied freely in any types.

FIGS. 9A-9C are diagrams illustrating laser beam characteristicsexperimentally shown according to the arrangement structure of the lasermaterial.

In common experimental conditions of FIGS. 9A-9C, the size of the lasermaterial is identical as 2×2×5 mm3 and a doping rate of Yb is 3%. Anoutput pulse power is 13˜15 W and a polarizing direction of a pump beamis substantially parallel to Nm-axes of the first laser material and thesecond laser material.

Based on such common experimental conditions, according to FIG. 9A, incase of the first laser material, a traveling direction of the laserbeam is substantially parallel to Ng-axis of the first laser materialand a polarizing direction (Elaser) of the laser beam is substantiallyparallel to Np-axis of the first laser material. Similar to the firstlaser material, the second laser material has beam characteristicsgenerated when a traveling direction of a laser beam is substantiallyparallel to Ng-axis of the second laser material and a polarizingdirection (Elaser) of a laser beam is substantially parallel to Nm-axisof the second laser material. As shown in the drawing, the shape of thelaser beam is oval, with beam distortion, and the beam quality might bedeteriorated accordingly.

According to FIG. 9A, in case of the first laser material, a travelingdirection of the laser beam is substantially parallel to Ng-axis of thefirst laser material and a polarizing direction (Elaser) of the laserbeam is substantially parallel to Nm-axis of the first laser material.Similar to the first laser material, the second laser material has beamcharacteristics generated when a traveling direction of a laser beam issubstantially parallel to Np-axis of the second laser material and apolarizing direction (Elaser) of a laser beam is substantially parallelto Nm-axis of the second laser material. As shown in the drawing, theshape of the laser beam is oval, with beam distortion, and the beamquality might be deteriorated accordingly.

According to FIG. 9C, in case of the first laser material, a travelingdirection of the laser beam is substantially parallel to Ng-axis of thefirst laser material and a polarizing direction (Elaser) of the laserbeam is substantially parallel to Np-axis of the first laser material.Similar to the first laser material, the second laser material has beamcharacteristics generated when a traveling direction of a laser beam issubstantially parallel to Np-axis of the second laser material and apolarizing direction (Elaser) of a laser beam is substantially parallelto Nm-axis of the second laser material. As shown in the drawing, theshape of the laser beam is almost circular, without the beam distortion,and the beam quality might be enhanced accordingly.

Meanwhile, it is preferred that a femtosecond laser having a highaverage power is used, considering a processing object material,environments of productive sites and stability of a laser system.Accordingly, the arrangement structure of laser materials may bedifferentiated to enhance the average power as shown in FIG. 10.

FIG. 10 is an optical conceptual diagram of a femtosecond laserapparatus to describe an arrangement structure of a laser material toenhance an average power of a laser beam according to another embodimentof the present invention.

FIG. 10 is similar to the optical conceptual diagram of FIG. 8illustrating the femtosecond laser apparatus. In other words, a secondlaser material (C2) may be arranged in a laser beam generating apparatusand the second laser material (C2) includes Np-axis substantiallyparallel to Ng-axis of the first laser material (C1), Ng-axissubstantially parallel to Np-axis of the first laser material (C1) andNm-axis substantially parallel to Nm-axis of the first laser material(C1).

The first laser material (C1) and the second laser material (C2) arearranged to make a traveling direction of the laser beams generated fromthe first laser material (C1) and the second laser material (C2) besubstantially parallel to Ng-axis of the first laser material andsubstantially parallel to Np-axis of the second laser material (C2).Also, the first laser material (C1) and the second laser material (C2)may be arranged to make a polarizing direction (Elaser) of the laserbeam be substantially parallel to Nm-axes of the first laser material(C1) and the second laser material (C2), respectively.

In other words, the first laser material (C1) and the second lasermaterial (C2) are arranged to increase emission cross sections of thefirst and second laser materials (C! and C2) and to enhance the averageoutput of the beams.

At this time, the first laser material (C10 and the second lasermaterial (C2) may be arranged to make a polarizing direction (Epump) ofa pump beam be substantially parallel to Nm-axes of the first and secondlaser materials (C1 and C2). Accordingly, an absorption cross sectionfor absorbing the pump beam generated from a laser diode may beincreased and laser beam generation efficiency may be enhanced.

A femtosecond laser apparatus according to one embodiment of the presentinvention shown in FIG. 10 may use three or more laser materials,similar to the embodiment of FIG. 8.

FIG. 11 is an optical conceptual diagram of another embodiment of thefemtosecond laser apparatus to describe an arrangement structure of alaser material provided in a laser beam generating apparatus of a maseroscillator or an amplifier according to one embodiment of the presentinvention.

The optical conceptual diagram of the femtosecond laser apparatus shownin FIG. 11 is similar to the optical conceptual diagram shown in FIG. 8and detailed description of identical characteristics will be omittedaccordingly.

As shown in FIG. 11, an optical component 10 may be arranged between afirst laser material (C1) and a second laser material (C2). The opticalcomponent 10 is arranged to make a polarizing direction of a laser beamgenerated from the first laser material (C1) and the second lasermaterial (C2) be substantially parallel to Np-axis of the first lasermaterial and Nm-axis of the second laser material (C2).

In other words, the optical component 10 may make the polarizingdirection of the laser beam be substantially parallel to a specificoptical axis of the laser material, to convert a spectrum bandwidth ofthe laser beam and to prevent beam distortion generated by a thermaleffect of the laser material.

At this time, the optical component 10 may include any types ofcomponents configured to convert the polarizing direction of the beam.Examples of the optical component 10 may include a polarizationconverter. Such a polarization converter includes half-wave plates,double Fresnel rhombs, broadband prismatic rotator, faraday rotator andcombination of mirrors.

Meanwhile, detailed description of laser beam generation efficiency,output compensation and the number of laser materials are identical tothe description of the optical conceptual diagram shown in FIG. 8, andthe detailed description thereof will be omitted accordingly.

Meanwhile, similar to the embodiment of FIG. 10, the polarizingdirection of the laser beam is adjusted by using the optical component,only to the average output of the beam can be enhanced.

FIG. 12 is an optical conceptual diagram of the embodiment of thefemtosecond laser apparatus to describe an arrangement structure of alaser material provided in a laser beam generating apparatus of a maseroscillator or an amplifier according to another embodiment of thepresent invention.

The optical conceptual diagram of the femtosecond laser apparatus shownin FIG. 12 is similar to the optical conceptual diagram of FIG. 10, anddescription of identical elements will be omitted accordingly.

As shown in FIG. 12, an optical component 10 may be arranged between afirst laser material (C1) and a second laser material (C2). The opticalcomponent 10 is arranged to make a polarizing direction of a laser beamgenerated from the first laser material (C1) and the second lasermaterial (C2) be substantially parallel to Nm-axis of the first lasermaterial and Nm-axis of the second laser material (C2).

The optical component 10 may make the polarizing direction of the laserbeam be substantially parallel to a specific optical axis of the lasermaterial, to increase emission cross section of the laser materials toenhance an average output of the beams.

At this time, the type and other characteristics of the opticalcomponent 10 are identical to those of the optical component 10according to the embodiment of FIG. 11.

FIG. 13 is a diagram illustrating a laser beam generating apparatusprovided in a master oscillator or amplifier according to one embodimentof the present invention. FIGS. 14A and 14B are diagrams illustratingbeam dumpers according to one embodiment of the present invention. FIG.14A is a plane diagram of a beam dumper and a partial sectional diagram.FIG. 14B is a front diagram of the beam dumper and a partial sectionaldiagram.

Referring to FIGS. 13, 14A and 14B, a laser beam generating apparatus200 includes a first laser material (C1) and a second laser material(C2) that are arranged, facing each other, and a beam dumper 30 arrangedbetween the first laser material (C1) and the second laser material(C2).

The beam dumper 30 includes a hole 33 formed in a central portionthereof and a beam absorbing part 32 arranged adjacent to the hole 33.

The hole 33 of the beam dumper is configured to pass a laser beamgenerated from the first and second laser materials there through. Thebeam absorption part 32 of the beam dumper is configured to cut off orabsorb pump beams failed to be absorbed to the first laser material (C1)and the second laser material (C2).

The pump beams generated from a laser diode (not shown) are absorbed tothe first and second laser materials (C1 and C2) and the other pumpbeams failed to be absorbed thereto are incident on an optical component(not shown) composing the laser apparatus and a mount (not shown)supporting the optical component. Accordingly, thermal distortion isgenerated in an overall portion of the laser apparatus and the beamalignment characteristic might be deteriorated.

The beam dumper 30 according to the embodiment of the present inventionabsorbs pump beams failed to be absorbed to the first laser material(C1) or the second laser material (C2) to prevent them from heatingcomponents composing the laser apparatus. Accordingly, the deteriorationof the beam alignment characteristic may be prevented.

In addition, a cold water channel 31 may be formed in the beam dumper 30to prevent the heat generated from the absorbed pump beams from beingtransmitted to neighboring areas. Cold water is supplied to the beamdumper 30 via the cold water channel 31 to chill the beam dumper 30. Inaddition, the cold water channel 31 may be longitudinally formed,arranged adjacent to the beam absorbing part 32 as closely as possible,to enhance chilling efficiency.

An insertion hole 25 may be formed in an upper portion of the beamdumper 30 to insert connection means therein and the connection meansmay be integrally coupled to an optical component or an optical mountthat composes the laser generating apparatus.

In case of it is mechanically connected with the other opticalcomponents, such the beam dumper 30 can prevent thermal distortion ofthe optical components generated in the process of generating oramplifying femtosecond pulses. In other words, the heat of the opticalcomponents generated by the pump beams may be transmitted and absorbedto the beam dumper such that the thermal distortion of the opticalcomponents can be prevented.

A screw fastening hole 36 may be formed in an upper lateral end of thebeam dumper 30 to fasten the beam dumper 30 to the connection means, incommunication with an optical mount (not shown).

The cold water channel 31 may be formed adjacent to the hole 33 or alonga circumference of the beam absorbing part 32 to enhance chillingefficiency.

The beam absorbing part 32 has a shape with a predetermined diametergetting reduced toward a central portion from both lateral ends of thebeam dumper 30. In other words, the beam absorbing part 32 has a conecross section to prevent the pump beam generated from the laser diodefrom being incident on and reflected by the beam dumper 30. The beamabsorbing part 32 may have any structures only if they can allow thebeam dumper to reduce reflectance of the pump beam.

Moreover, a surface of the beam absorbing part 32 may be anodized orcoated with a material having a high beam absorption coefficient, toenhance a beam absorption coefficient.

Next, an experimental embodiment using the femtosecond laser apparatusaccording to the present invention and the femtosecond laser systemincluding the femtosecond laser apparatus will be described as follows.

Experimental Embodiment

The femtosecond master oscillator 110 is fabricated to generate seedingpulses which will be applied to an amplifier 170.

At this time, the mater oscillator 110 is fabricated of a Yb:KYW lasermaterial having the size of 3×3×2 mm3 and a Yb doping ratio of 5 at. %

A polarizing direction of the pump beam is substantially parallel toNm-axis of the laser material and an oscillation polarizing direction ofa laser is also substantially parallel to Nm-axis.

To make a central wavelength of the oscillating femtosecond materoscillator 110 fitted to a central wavelength of a resonance cavityprovided in an amplifier 170, the laser material may be Np-cut of thelaser material in an Np-axis direction.

The central wavelength of the laser beam output from the materoscillator 110 is 1035 nm and the spectrum bandwidth of the laser beamis 9.0 nm. The pulse time width of the laser beam is 110 fs and theaverage output of the laser beam is 1.2 W.

The pulse stretcher 131 is a device configured to temporally stretch thelength of the pulse before the pulse is amplified by the amplifier 170.The pulse compressor 132 is a device configured to return thelongitudinally stretched time width of the pulse to the short time widthof the pulse in the femtosecond range.

Once the pulse having the time width longitudinally stretched hundredsof or thousands of times or more is amplified by the amplifier 170 insuch a chirp pulse amplification process, a peak power of the amplifiedpulse is lowered and physical damage to the optical components composingthe resonance cavity of the amplifier 170 may be prevented.

Moreover, the temporal shape distortion of the pulse and the spatialdistribution distortion of the beam can be prevented may be prevented bya non-linear phenomenon and examples of such a non-linear phenomenoninclude a self-focusing effect generated in a high peak power.

This experiment uses one transmission diffraction grating having agroove density of 1500 lines/mm designed and fabricated to be employedas the pulse stretcher 131 and the pulse compressor 132 simultaneously.

The results of a test performed for the pulse stretcher 131 and thepulse compressor 132 by using the femtosecond pulse of the materoscillator 110 will be as follows.

The femtosecond pulse having a time width of 110 fs is stretched into apulse of approximately 50 ps by the pulse stretcher 131. When thestretched pulse passes the pulse compressor 132, the pulse is compressedinto a pulse of 160 fs.

A compression rate showing a rate of the pulse time width beforestretched to the pulse time width after stretched is 1.45. In addition,a power conversion efficiency of the pulse between before and afterpassing the pulse stretcher 131 is 74% and a power conversion efficiencyof the pulse between before and after passing the pulse compressor 132is 78%.

Only desired ones of the pulses emitted outside can pass the pulsepicker 160.

At this time, the pulse picker 160 divides main pulses into pre-pulsesand post-pulses.

Meanwhile, two thin film polariscopes are provided in the amplifier 170to enhance a relative contrast ratio between laser pulses desired toemit outside the resonance cavity and weak laser pulses leak out of theresonance cavity.

An auxiliary thin film polariscope is arranged in an entire system toenhance the final contrast ratio.

The amplifier 170 uses a Yb:KYW laser material having a size of 2×2×5mm3 and a Yb3+ ion doping concentration of 3 at. %. Non-coating withrespect to the pump beam and the laser oscillation wavelength isperformed on both ends of the laser material.

A dichromatic plane mirror (DM) is coated to pass pump beams in awavelength range of 981 nm there through at a high transmittance and toreflect lasers in a wavelength range of 1 micrometer at a highreflectance.

As the pump beam source, two laser diodes 21 having a high brightnesswith a wavelength of 981 nm and the maximum power of 70 W are arrangedin the laser materials (C1 and C2), respectively.

Yb:KYW used in this experiment is an anisotropic laser material that hasdifferent characteristics according to an optical axial direction.

Accordingly, the optical fiber 22 connected to the laser diode 21 isshortened as much as possible to maintain a polarizing direction of thepump beam.

This experiment uses a high brightness laser diode 21 having a length of30 cm, a core diameter of 200 μm and a numerical aperture (NA) of 0.22.

To make the pump beams absorbed to the laser material as many aspossible, a half-wave plate λ/2 is arranged next to the optical fiber 22to adjust the polarizing direction microprecisely, such that thepolarization of the pump beam may be substantially parallel to Nm-axisof Yb:KYW crystal.

The laser material (C1) is Ng-cut cut in an Ng-axial direction and apolarizing direction of the pump beam is substantially parallel toNm-axis and a polarizing direction of the laser beam is substantiallyparallel to Np-axis.

The laser material (C2) is Np-cut cut in an Np-axial direction and apolarizing direction of the pump beam is substantially parallel toNm-axis and a polarizing direction of the laser beam is alsosubstantially parallel to Nm-axis.

In other words, the polarizing directions of the pump beams aresubstantially parallel to Nm-axis having the largest absorption crosssection. One polarizing direction of the laser beam is substantiallyparallel to Nm-axis having the largest emission cross section and theother one is substantially parallel to Np-axis, to combine differentgain spectrums with each other.

The combination between one Ng-cut laser material and the other Np-cutlaser material makes laser oscillation combine different gain spectrumdistributions, such that the bandwidth may be increased to pulse ashorter femtosecond pulse and that the polarizing directions of thelaser beams may be toward axial directions having different terminalcharacteristics. Accordingly, distribute the thermal effect.

The length of the laser material is 5 mm that is relatively long and thedoping rate is 3 at. %, to reduce a thermal lens effect and to enhance aspatial quality of the output beams.

According to experimental results supporting that, there may be usedLASCAD software (Las-CAD GmbH) capable of implementing numericalsimulation of power characteristics by configurating a resonance cavityvirtually. For example, a thermal lens of and an optical strength of athermal-mechanical stress on Yb:KYW laser crystal having a length of 5mm and a doping rate of 3 at. % may be 1.5 times weaker than a thermallens of and an optical strength of a thermal-mechanical stress on alaser crystal having a length of 3 mm and a doping rate of 5 at. %

Moreover, computer calculating shows the astigmatism of the Ng-cutcrystal is similar to that of the Np-cut crystal and that the axialdirection of the Ng-cut crystal is different from that of the Np-cutcrystal.

When the power of the pump beam applied to the laser material is 36 W, aratio between thermal focus distances in x-axis and y-axis directions(fx/fy) is 1.15 in case of the Ng-cut and 0.88 in case of the Np-cut.

That means that the astigmatism of the amplified beams can be partiallybalanced in case the laser beam passes the Ng-cut crystal and the Np-cutcrystal serially or in case the laser beam passes the Np-cut crystal andthe Ng-cut crystal serially.

Meanwhile, in the experiment according to the embodiment of the presentinvention, the doping concentration of Yb is preset as a specific value.However, the Yb doping concentration may be in a range of 1˜10 at. %where the power intensity of the laser beam and the quality of the laserbeam can be enhanced.

Next, experimental results of a femtosecond laser apparatus and afemtosecond laser system including the femtosecond laser apparatusaccording to one embodiment of the present invention will be describedas follows.

Experimental Results

FIG. 15 shows operational characteristics of a continuous wave (CW)output power according to an incident pump power on crystals of FIG. 7.

First of all, a pump beam is applied to only of laser materials havingNg-cut and Np-cut and slope efficiencies are 47% and 37%, respectively.

At this time, when the applied pump beam is 36 W, the maximum powers are12 W and 9 W.

When pump beams are applied to the laser materials having Ng-cut andNp-cut, the slop efficiency is approximately 35%. When the applied pumpbeam is 72 W, a continuous wave power of an amplifier 170 is 18 W.

When a gate time is 800 ns and a pulse repetition rate is 200 kHz inQ-switch mode, an average power of 16 W is gained.

The reason why the power is reduced, compared with a continuous wavemode, is that the power loss is generated by an optical switch arrangedin a resonance cavity.

The pulse time width is approximately 20 ns and the pulse spectrumbandwidth is approximately 16 nm.

The pulse spectrum shows an M-shape having two peaks of 1035 nm and 1043nm according to different gain peaks possessed by two laser materials.

The spectrum narrowing is generated in a process of amplifying the pulseenergy performed by the amplifier 170. To restrain the spectrumnarrowing and to broaden the spectrum bandwidth, apolarization-interference filter called ‘Lyot filter’ is provided toperform spectral shaping is performed in an extra-cavity orintra-cavity.

Meanwhile, a spectral shaper 140 is configured of two polarizing plates122 and a birefringent quartz plate 121 arranged between the polarizingplates 122 as shown in FIG. 16.

The minimum transmittance point of the birefringent filter has tocorrespond to the maximum point of the gain spectrum and the widthsthereof have to be similar to each other.

To realize that, a quartz plate having a thickness of 8 mm is cut alongan optical axis and the cut quartz plate is mounted to a rotation mount123 to rotate in Φ and Ψ rotational directions precisely, such that theposition and the modulation depth of the minimum transmittance point maybe adjusted.

Lastly, a seeding pulse having the pulse stretching and the spectrumshaping is applied to the amplifier 170 and the overall system shown inFIG. 1 is configurated. After that, operational characteristics of thesystem are measured.

FIG. 17 is a graph illustrating a spectrum before (a) and after (b)spectrum-shaping a pulse chirped by the pulse stretcher 131 according toone embodiment of the present invention.

The pulse emitted from the mater oscillator 110 of FIG. 1 has asymmetrical spectrum having a central wavelength of 1035 nm and abandwidth of 9 nm.

After spectrum-shaping, a local maximum value is calculated near 1030 nmand 1040 nm.

The thickness and rotational direction (Φ and Ψ) of the quartz platecomposing the spectral shaper 140 are adjusted and various spectrumshaping types are possible.

(a) of FIG. 18 shows a non-symmetrical spectrum having a centralwavelength of 1036 nm and a bandwidth of 6 nm that is a measuredspectrum of a laser pulse amplified when a seeding pulse notspectrum-shaped is applied to an amplifier 170 and when pump beamshaving the same intensity to laser crystals (C1 and C2) at a pulserepetition rate of 200 kHz.

Compared with (a) of FIG. 17 showing a pulse spectrum before amplified,the spectrum bandwidth is narrowed from 9 nm to 6 nm by the gainnarrowing.

When the narrowed spectrum pulse is compressed by a pulse compressor132, the pulse time width measured as 265 fs as shown in (a) of FIG. 19.

Such the spectrum narrowing may be retrained by various methods.

For example, pump beams having different intensities, respectively, areapplied to C1 and C2 of Yb:KYW that is an anisotropic laser crystal, thepump beams have different maximum gain values at central wavelengthsonly to gain an effect of combining gain spectrums having differentintensities.

In an actual experiment, a rate of pump beam powers applied to Np-cutcrystal and Ng-cut crystal is changed into 3:2, the spectrum may bebroadened and the shape of the spectrum may be distorted.

In this instance, the spectrum bandwidth is 9 nm and the pulse timewidth is 210 fs. Then, the narrowed pulse time width can be gained.However, the intensity of the pump power is limited and an overall powerof the femtosecond laser system is limited disadvantageously.

This experiment measures that the power is reduced by 37% under theconditions mentioned above.

A further method for retraining the spectrum narrowing is to usespectrum-shaping. The spectral shaper 140 shown in FIG. 16 is arrangedbetween the pulse stretcher 131 and the amplifier 170 shown in FIG. 1,to implement spectrum shaping of the seeding pulse in an extra-resonancecavity.

An angle (Φ and Ψ) of a rotation mount 123 is precisely adjusted and thespectrum of the amplified pulse and the time width of the pulse aremeasured to determine an optimal angle.

(b) of FIG. 18 shows a spectrum of a laser pulse amplified when aseeding pulse is spectrum-shaped.

The measured spectrum has a bell shape, with a central wavelength of1034 nm and a spectrum band width of 11 nm.

The band width of the amplified pulse is 6 nm before spectrum-shapingand the band width is 11 nm after the spectrum-shaping, which is almosttwice larger. When the broadened spectrum pulse is compressed by a pulsecompressor 132, the measured pulse time width is 182 fs as shown in (b)of FIG. 19.

The experiment is performed when the spectral shaper 140 is arranged ina resonance cavity of the amplifier 170.

The spectrum having almost the same area can be gained and the laserpower is reduced approximately 20%.

That is because small loss generated by the spectral shaper 140 having aLyot filter accumulates while a laser pulse reciprocates within theresonance cavity.

FIG. 20 is a diagram illustrating pulse energy change according to apulse repetition rate.

As the pulse repetition rate is getting lower, the pulse energy isgetting higher. The maximum pulse energy measured when the pump powerapplied to the laser crystal is 73.2 W and the pulse repetition rate is50 kHz may be 165 μJ.

In the experiment, it is observed that Raman scattering generated at alower pulse repetition rate and Raman scattering may be a factorinterfering with enhancement of pulse energy.

A pulse energy of 10˜15 μJ is gained at a pulse repetition rate of 200kHz and a pulse energy of 4˜20 μJ is gained at a pulse repetition rateof 500 kHz.

Such the pulse energy is sufficient to process various sample materialsat a high pulse repetition rate.

Various variations and modifications of the femtosecond laser apparatusand the femtosecond laser system including the femtosecond laserapparatus are possible in the component parts and/or arrangements of thesubject combination arrangement within the scope of the disclosure, thedrawings and the appended claims. In addition to variations andmodifications in the component parts and/or arrangements, alternativeuses will also be apparent to those skilled in the art.

What is claimed is:
 1. A femtosecond laser apparatus comprising: a firstlaser material comprising Ng, Np and Nm axes spatially perpendicular toeach other; a second laser material comprising Np axis substantiallyparallel to Ng axis of the first laser material, Nm axis substantiallyparallel to Np axis of the first laser material and Ng axissubstantially parallel to Nm axis of the first laser material; and afirst laser diode and second laser diodes arranged to irradiate pumpbeams to the first laser material and the second laser material,respectively, wherein the first laser material is arranged to make atraveling direction of laser beams generated from the first and secondlaser materials be substantially parallel to Ng axis of the first lasermaterial and to make a polarizing direction of laser beams generatedfrom the first and second laser materials be substantially parallel toNp axis of the first laser material, and the second laser material isarranged to make a traveling direction of laser beams generated from thefirst and second laser materials be substantially parallel to Np axis ofthe second material and to make a polarizing direction of laser beamsgenerated from the first and second laser materials be substantiallyparallel to Nm axis of the second laser material.
 2. The femtosecondlaser apparatus according to claim 1, wherein each of the first andsecond laser materials is at least one of Yb:KYW, Yb:KGW, Yb:KLuW,Yb:YCOB and Yb:YAP.
 3. The femtosecond laser apparatus according toclaim 2, wherein an Yb doping concentration of Yb:KYW, Yb:KGW, Yb:KLuW,Yb:YCOB and Yb:YAP is 1˜10 at. %.
 4. The femtosecond laser apparatusaccording to claim 1, wherein the first laser material is configured tomake a polarizing direction of a first pump beam generated from thefirst laser diode be substantially parallel to Nm axis of the firstlaser material, and the second laser material is configured to make apolarizing direction of a second pump beam generated from the secondlaser diode be substantially parallel to Nm axis of the second lasermaterial.
 5. A femtosecond laser apparatus comprising: a first lasermaterial comprising Ng, Np and Nm axes spatially perpendicular to eachother; a second laser material comprising Np axis substantially parallelto Ng axis of the first laser, Ng axis substantially parallel to Np axisof the first laser material and Nm axis substantially parallel to Nmaxis of the first laser material; and a first laser diode and secondlaser diodes arranged to irradiate pump beams to the first lasermaterial and the second laser material, respectively, wherein the firstlaser material is arranged to make a traveling direction of laser beamsgenerated from the first and second laser materials be substantiallyparallel to Ng axis of the first laser material and to make a polarizingdirection of laser beams generated from the first and second lasermaterials be substantially parallel to Nm axis of the first lasermaterial, and the second laser material is arranged to make a travelingdirection of laser beams generated from the first and second lasermaterials be substantially parallel to Np axis of the second materialand to make a polarizing direction of laser beams generated from thefirst and second laser materials be substantially parallel to Nm axis ofthe second laser material.
 6. A femtosecond laser system comprising: amaster oscillator configured to generate a pulse in a femtosecond range;a pulse stretcher configured to stretch a width of the generated pulse;an amplifier configured to amplify an energy of the stretched pulse; anda pulse compressor configured to compress the amplified pulse into apulse in a femtosecond range, wherein at least one of the masteroscillator and the amplifier comprises the laser apparatus recited inclaim
 1. 7. The femtosecond laser system according to claim 6, whereinthe pulse stretcher and the pulse compressor are integrally formed viaone spectral device commonly shared by the pulse stretcher and the pulsecompressor.
 8. The femtosecond laser system according to claim 6,further comprising: a spectral shaper configured to shape a spectrum ofthe stretched pulse.
 9. A femtosecond laser apparatus comprising: afirst laser material and a second laser material comprising Ng, Np andNm axes, respectively, that are spatially perpendicular to each other,the first laser material and the second laser material facing eachother; a first laser diode and a second laser diode arranged to makepump beams incident on the first laser material and the second lasermaterial, respectively; and an optical component arranged between thefirst laser material and the second material to make a polarizingdirection of laser beams generated from the first and second lasermaterials be substantially parallel to Np axis of the first lasermaterial and to Nm axis of the second laser material, wherein the firstlaser material is arranged to make a traveling direction of laser beamsgenerated from the first and second laser materials be substantiallyparallel to Ng axis of the first laser material, and the second lasermaterial is arranged to make a traveling direction of laser beamsgenerated from the first and second laser materials be substantiallyparallel to Np axis of the second laser material.
 10. The femtosecondlaser apparatus according to claim 9, wherein the optical component isone of half-wave plates, double Fresnel rhombs, broadband prismaticrotator, faraday rotator and combination of mirrors.
 11. The femtosecondlaser apparatus according to claim 9, wherein each of the first andsecond laser materials is at least one of Yb:KYW, Yb:KGW, Yb:KLuW,Yb:YCOB and Yb:YAP.
 12. The femtosecond laser apparatus according toclaim 11, wherein an Yb doping concentration of Yb:KYW, Yb:KGW, Yb:KLuW,Yb:YCOB and Yb:YAP is 1˜10 at. %.
 13. The femtosecond laser apparatusaccording to claim 9, wherein the first laser material makes apolarizing direction of a first pump beam generated from the first laserdiode be substantially parallel to Nm axis of the first laser material,and the second laser material makes a polarizing direction of a secondpump beam generated from the second laser diode be substantiallyparallel to Nm axis of the second laser material.
 14. A femtosecondlaser apparatus comprising: a first laser material and a second lasermaterial comprising Ng, Np and Nm axes, respectively, that are spatiallyperpendicular to each other, the first laser material and the secondlaser material facing each other; a first laser diode and a second laserdiode arranged to make pump beams incident on the first laser materialand the second laser material, respectively; and an optical componentarranged between the first laser material and the second material tomake a polarizing direction of laser beams generated from the first andsecond laser materials be substantially parallel to Nm axis of the firstlaser material and to Nm axis of the second laser material, wherein thefirst laser material is arranged to make a traveling direction of laserbeams generated from the first and second laser materials besubstantially parallel to Ng axis of the first laser material, and thesecond laser material is arranged to make a traveling direction of laserbeams generated from the first and second laser materials besubstantially parallel to Np axis of the second laser material.
 15. Afemtosecond laser system comprising: a master oscillator configured togenerate a pulse in a femtosecond range; a pulse stretcher configured tostretch the width of the generated pulse; an amplifier configured toamplify an energy of the stretched pulse; and a pulse compressorconfigured to compress the amplified pulse into a pulse in thefemtosecond range, wherein at least one of the master oscillator and theamplifier comprises the laser apparatus recited in claim
 9. 16. Thefemtosecond laser system according to claim 15, wherein the pulsestretcher and the pulse compressor are integrally formed via onespectral device commonly shared by the pulse stretcher and the pulsecompressor.
 17. The femtosecond laser system according to claim 15,further comprising: a spectral shaper configured to shape a spectrum ofthe stretched pulse.
 18. A femtosecond laser apparatus comprising: afirst laser material and a second laser material facing each other; afirst laser diode and a second laser diode arranged to make pump beamsincident on the first laser material and the second laser material,respectively; and a beam dumper arranged between the first lasermaterial and the second laser material, the beam dumper comprising ahole to pass laser beams generated from the first and second lasermaterials therethrough and a beam absorbing part formed adjacent to thehole to cut off or absorb the pump beams failed to be absorbed to thefirst and second laser materials.
 19. The femtosecond laser apparatusaccording to claim 18, wherein the beam absorbing part has a diametergetting smaller to a central portion from both lateral ends of the beamdumper.
 20. The femtosecond laser apparatus according to claim 18,wherein the beam absorbing part is anodized.
 21. The femtosecond laserapparatus according to claim 18, wherein the beam dumper comprises acold water channel provided therein.
 22. The femtosecond laser apparatusaccording to claim 18, wherein the beam dumper comprises an insertionhole and connection means is inserted in the insertion hole to make thebeam dumper integrally formed with optical components.
 23. Thefemtosecond laser apparatus according to claim 21, wherein the coldwater channel is arranged adjacent to the beam absorbing part.
 24. Afemtosecond laser system comprising: a master oscillator configured togenerate a pulse in a femtosecond range; a pulse stretcher configured tostretch the width of the generated pulse; an amplifier configured toamplify an energy of the stretched pulse; and a pulse compressorconfigured to compress the amplified pulse into a pulse in thefemtosecond range, wherein at least one of the master oscillator and theamplifier comprises the laser apparatus recited in claim
 18. 25. Thefemtosecond laser system according to claim 24, wherein the pulsestretcher and the pulse compressor are integrally formed via onespectral device commonly shared by the pulse stretcher and the pulsecompressor.
 26. The femtosecond laser system according to claim 24,further comprising: a spectral shaper configured to shape a spectrum ofthe stretched pulse.